Royal right … Saudi King Abdullah in conversation with Bahrain's king during a festival in April showcasing Saudi Arabian culture. Photograph: Str/Reuters

One of the less-discussed facts about the wave of uprisings in the Middle East is that the Arab monarchies are still relatively unscathed. The regimes most seriously challenged by popular protests – in Tunisia, Egypt, Yemen, Libya and Syria – have all been republics. This may seem odd to Europeans whose revolutions over the centuries have been mainly about overthrowing kings.

To some extent, the apparent resilience of Arab monarchies may be a matter of luck. Most of them are in the Gulf and they have oil, which means they can (and do) use their money to buy off discontent. That does not apply to the kingdoms of Jordan and Morocco, however, and oil wealth has not saved the Gaddafi regime from trouble in the Great Socialist People's Libyan Arab Jamahiriya.

Another possible explanation is that Arab monarchs, in the eyes of many of their citizens, have a stronger claim to legitimacy than republican leaders who came to power – or clung on to it – in dubious circumstances.

The monarchies base their legitimacy on religious or tribal roots. The rulers of Kuwait, Qatar, Oman, Bahrain and the Emirates all came from old and prominent tribes and the "right" to rule was derived from their families' status.

The Sabah family, for instance, was a clan of the Anizah tribe which migrated from Nejd – the central plateau of Saudi Arabia – to Kuwait in the 18th century and has ruled locally ever since. The Khalifa family was another clan from the same tribe that had arrived in Bahrain about the same time. The Thani family that rules Qatar is a branch of the Bani Tameem tribe and also arrived from Nejd in the 18th century.

The Saudi royal family has tribal roots too, though its main claim to legitimacy today is religious – so much so that the king's religious title, Guardian of the Two Holy Shrines (Mecca and Medina, the two holiest sites in Islam) takes precedence over his royal title.

Similarly, the king of Jordan is official guardian of al-Aqsa mosque in Jerusalem, regarded as Islam's third holiest site. Jordan's current monarch, Abdullah II, also boasts of being a "43rd generation direct descendant of the Prophet Muhammad". Meanwhile the king of Morocco embodies both "spiritual and temporal authority" and is known as Amir al-Mu'mineen – the prince (or commander) of the believers.

Although rule by birthright might seem an inherently objectionable form of government, the tribal and religious background makes it difficult to challenge in what are often highly traditional and patriarchal societies. In the monarchies where there have been significant protests, such as Morocco, Oman and Jordan, demonstrators have been demanding reform but without questioning the ruler's right to govern – which is still very much a taboo. (Bahrain is a special case, where a Sunni Muslim minority rules over a Shia majority, making the legitimacy question much more obvious.)

While the legitimacy claims of Arab monarchs might not seem particularly convincing, especially to outsiders, those of the republics are even less so.

A number of revolutionary Arab regimes emerged in the 20th century whose credentials were based primarily on nationalism: Algeria, Egypt, Iraq, Libya, Syria, Tunisia, the separate states of North and South Yemen – plus the Palestinian liberation movement, which fitted a similar mould.

Typically, these revolutionary regimes pursued populist or socialist strategies – nationalisation, land reform and so on – which held out the promise of a better future for the masses. At the same time, they presented themselves as defenders of the nation's independence, resisting the corrupting, exploitative effects of western imperialism and in particular generating unfulfillable popular expectations regarding the conflict with Israel.

In the wake of successive defeats by Israel, and amid high unemployment, poverty and rampant corruption, it became all too obvious that they were failing to deliver.

Some of the republican regimes further undermined their credibility by starting to resemble monarchies. It began in 2000, when Bashar al-Assad inherited the Syrian presidency from his father. The dictators of Egypt, Libya, Iraq, Tunisia and Yemen also showed signs of intending to hand over power, eventually, to sons or other relatives.

Arabs mockingly combined the words for "republic" and "monarchy" to coin a new term for this type of state: jumlukiyya.

The republics – and especially the jumlukiyyas – thus found themselves scrabbling around for reasons to justify their existence. The problem was apparent even in 2004 when the UN's Arab Human Development report spoke of a "crisis of legitimacy":

"Most regimes, nowadays, bolster their legitimacy by adopting a simplified and efficient formula to justify their continuation in power. They style themselves as the lesser of two evils, or the last line of defence against fundamentalist tyranny or, even more dramatically, against chaos and the collapse of the state … "

"Sometimes," the report said, "the mere preservation of the state entity in the face of external threats was considered an achievement sufficient to confer legitimacy."

Strangely, it does not seem to have occurred to them that there was one way they might have re-established their legitimacy: by governing the country justly and well.

So it's not very surprising that the regimes already toppled or currently under threat are republics of the family-run jumlukiyya variety. This does not mean the others are immune – and it's worth recalling monarchs were overthrown in Egypt, Yemen and Libya during the 1950s and 1960s.

For now, though, the remaining monarchs are sitting on their thrones fairly comfortably. After a rocky moment, even the king of Bahrain seems to have won more time in power, thanks to support from the royals in neighbouring countries.

This gives them a breathing space in which to reform – if they choose to do so. Whether they will seize the opportunity is another matter. At present, Morocco and Kuwait are the only two that look as if they might, possibly, turn into constitutional monarchies with accountable government. But if they don't change, their turn will surely come.